The Federal Register vol. 55 No. 61 pp. 11863-875 discloses a zero-headspace extractor vessel which provides for liquid/solid separation within the vessel and effectively or substantially precludes headspace.
The specification set forth in this Federal Register citation includes details of extraction methods as well as schematic drawings of typical specified apparatus. One such drawing is reproduced in FIG. 1 of the accompanying drawings where it is labeled "prior art".
The Federal Register prior art will be described in more detail hereinbelow. At this juncture, it will suffice to say that the zero-headspace extractor vessel shown comprises a cylinder which is divided, in a lengthwise sense, by a slidable piston into a sample chamber having an access port and a pressurizing chamber equipped with pressuring means. The access port is closable by solids-separation or filtration means and the pressurizing means is operable to drive the piston in the cylinder and press the sample mixture out through the filtration means where solids are retained and a liquid filtrate is expelled.
According to the drawing, the pressurization means can be a pressurized gas, controlled by a valve and the text states that some other zero-headspace extractors use mechanical pressure. Five such devices are referenced by manufacturer. A particular specification of interest in practicing the present invention calls for a vessel capacity of 500 to 600 ml., the ability to operate at a pressure of 50 psig, for a piston equipped with seals that do not leak at 50 psig and which will slide under an applied pressure of 15 psi, or less.
Because zero-headspace extractors are used to prepare specimens for sensitive tests of trace contaminants, there are precise requirements for their manufacture. Some of these requirements concern the materials of construction.
The Federal Register states that extraction vessels and filtration devices shall be made of inert materials which will not leach or absorb waste components. Specifically, "glass, polytetrafluoroethylene (PTFE) or type 316 stainless steel equipment may be used when evaluating the mobility of both organic and inorganic components. Devices made of high-density polyethylene (HDPE), polypropylene or polyvinyl chloride may be used only when evaluating the mobility of metals." Elsewhere, it is made clear that plastic materials, other than PTFE, cannot be used for determining organics.
Although not explicitly stated in this reference, stainless steel is not ideal for heavy metal determinations since it can leach contaminants such as chromium and nickel, from the vessel, into strong acids such as nitric that may be found in the sample. Furthermore, stainless steel is expensive and dense, rendering the extractor cumbersome and placing excess loads on tumblers or rotators used to agitate such vessels charged with sample mixtures. Stainless steel extraction vessels of the specified capacity of around 500-600 ml. can weigh as much as 15 lb. and usually are inconvenient to use, lacking a flat base on which they can be set for charging or discharging. Glass, especially the recommended borosilicate glass is excellent for inertness to contaminants, but is of course breakable. PTFE lacks structural strength and dimensional stability, is hard to shape and is expensive. While the other polymers mentioned in the Federal Register reference are excellent lightweight structural materials, they suffer the limitation of being usable only for metal determinations.
In developing this invention, applicants have undertaken the expense of a patent search of the prior art, which search revealed no relevant publications. By way of general background, the search revealed U.S. Pat. No. 632,830 (Betzonick), and U.S. Pat. No. 3,866,860 (Boehringer).
Betzonic employs a screw press to drive juice out of a sample of citrus fruit, through a crude, built-in sieve or filter. Boehringer discloses a gas-sampling device which provides a zero-headspace container for the gas for transport which container can be punctured to release the gas for analysis.
Two further U.S. patents disclosing zero headspace devices are U.S. Pat. No. 4,864,877 and U.S. Pat. No. 4,974,566 (both to Ortiz et al.). These patents, which have related disclosures, the latter being a divisional of the former, disclose zero-headspace sampling containers intended to meet EPA sampling requirements for liquids. Ortiz et al. do not disclose filtration means to retain sediment and do not appear to disclose pressurization of the containers. None of the constructions of these prior patents is suitable for meeting the objects of the present invention.